Specifying a locator object via a kwarg to contourf results in levels coming from a call to tick_values, which bypasses the view_limits calculation. This means that setting the 'symmetric =True' kwarg in a MaxNLocator passed to contourf has no effect. One way to fix this would be to revert 354d133, which removed the use of a dummy axis in contourf. Another way would be to use the symmetric setting in tick_values itself. A third would be to allow the Locator.__call__ method to take vmin, vmax arguments, avoiding the need to explicitly create a dummy axis and set its limits.

My inclination is to do the quick fix--revert 354d133--in v2.x, and then switch to something like the third option for 2.1 or later.